The present invention is directed to an RF amplifier system and, more particularly, to such a system employing pulse duration modulation (PDM) drive for actuating transistor switches.
It is frequently desirable that transistor switches, such as MOSFET transistor switches, be connected in a series circuit for purposes of connecting a high voltage source, such as 50 kV across a load at a radio frequency, such as in a range from 100 KHz to 2 MHz. An example is a carrier wave within this frequency range, such as at 1 MHz, and having a frequency band on the order of 20 KHz to accommodate audio frequency signals from approximately 0 to 20 KHz.
In the case of MOSFETs, the voltage breakdown of each MOSFET is on the order of 500 V. If one hundred MOSFETs are connected together in series, the total circuit withstanding voltage is on the order of 50 kV.
The need arises as to how to control the various MOSFETs. Pulse duration modulation (PDM) has been employed in RF amplifiers in the past. Examples are found in the U.S. patents to H. I. Swanson U.S. Pat. Nos. 3,506,920 and 4,164,714. However, these patents do not disclose PDM control of a plurality of series connected switching transistors for connecting a voltage source across a load at a high frequency.
In accordance with the invention, an RF amplifier system includes a plurality of actuatable switches each switch being normally OFF and being actuated to an ON condition by a received turn ON pulse and then remains ON for the duration of the turn ON pulse. The switches are connected in a series circuit with a load across a DC voltage source. An RF source provides a train of RF pulses exhibiting a fixed frequency and fixed duty-cycle and wherein each RF pulse is of a fixed magnitude and duration. A control circuit receives the RF pulses and provides therefrom the turn ON pulses which are of equal duration and which are synchronized to the RF pulses. The control circuit simultaneously applies the turn ON pulses to the plurality of transistor switches. The control circuit includes a pulse duration modulator that varies the duration of the turn ON pulses.
In accordance with a more limited aspect of the present invention, the train of RF pulses is a first train of first RF pulses and the modulator includes a phase shifter that receives the first RF pulses and provides therefrom a second train of second RF pulses identical to the first train of RF pulses but phase shifted therefrom.
In accordance with a still further aspect of the present invention, the system includes a pulse magnitude adjuster that adjusts the relative magnitude between the first and second trains of RF pulses so that the first RF pulses are of a different magnitude than the second RF pulses.
In accordance with a still further aspect of the present invention, the system includes a pulse combiner that combines the first and second RF pulses to provide the turn ON pulses each having a duration dependent upon the phase relationship between the first and second pulse trains.